As display devices, those of the CRT type wherein phosphors are irradiated with high-speed electron beams for excitation are the most excellent from the viewpoint of the quality of images. However, television sets of the CRT type, when having a large screen, exceed 170 kg in weight and 850 mm in depth and are therefore not acceptable generally for household use.
Accordingly, flat displays of the electron beam type are proposed in U.S. Pat. No. 4,719,388 or Unexamined Japanese Patent Publication SHO 61-242489, and like publication SHO 62-90831. The proposed displays have a cathode of linear filaments as an electron beam emitter and XY matrix electrodes for withdrawing high-speed electron beams, which are caused to impinge on a fluorescent screen at specified addresses.
With reference to FIGS. 1 and 2, the flat display comprises a front panel 1 having a fluorescent screen 10 on its rear surface, and a rear panel 2 having a back electrode 20 on its inner surface and defining a flat hermetic space together with the panel 1. An address electrode board 4 and a grid electrode 5 provided with a gridded surface 50 are arranged in the space in parallel to the panels. The address electrode board 4 comprises first address electrodes 42 arranged on one surface of a substrate 40 and extending in one direction of an XY matrix, and second address electrodes 44 arranged on the other surface of the substrate 40 and extending in a direction intersecting the first address electrodes 42 at right angles therewith. The points where the first address electrodes 42 intersect the second address electrodes are each formed with one or more than one aperture 41. The two groups of address electrodes of the display are controlled by electrode control-drive circuits 6, 7, respectively, as will be described later. When a positive voltage is applied to one selected second address electrodes 44 extending in X-direction and to the first address electrodes 42 extending in Y-direction at the same time, electron beams are drawn through the apertures 41 positioned at the points of intersection of these electrodes to irradiate the phosphor dots at the specified addresses on the fluorescent screen on the front panel 1 to which a high voltage is applied, causing the dots to luminesce.
Since the fluorescent screen of the flat display described is excited basically on the same principle as the CRT, the flat display of this type has the advantage of giving images of higher quality than flat displays of other types, such as the PDP (plasma display panel) type, LCD (liquid crystal display) type, VFT (fluorescent display tube) type, etc.
The luminance of the screen is increased by various contrivances, for example, by enlarging the apertures of the address electrode board 4 to pass larger quantities of beams therethrough, or by applying a higher voltage to the address electrodes 42, 44 to draw electrons from the cathode with greater ease.
FIG. 3 shows the configuration and arrangement of the address electrodes. For example, when the second address electrode 44 disposed on the cathode side are the scanning electrodes, the first address electrodes 42 arranged on the fluorescent screen side serve as data-side electrodes to which an image signal is applied.
The fluorescent screen 10 has phosphor dots 11 which are arranged usually in a delta pattern, and the apertures 41 are formed in corresponding relation to the respective dots.
With reference to FIG. 3, the second address electrodes 44 are represented one after another by X.sub.1, . . . , X.sub.n, X.sub.n+1, . . . , and the first address electrodes 42 by Y.sub.1, . . . , Y.sub.m, Y.sub.m+1, Y.sub.m+2, Y.sub.m+3, Y.sub.m+4, . . . . As shown in FIG. 8, a scanning signal voltage 70 is applied to the second address electrode X.sub.n during one period H of horizontal scanning, whereupon the voltage is applied to the second address electrode X.sub.n+1 during the next period H.
In the case where the image data signal is quantized and subjected to pulse-width modulation for the first address electrodes 42, the image data signal stored in a shift register and latch of the data-side electrode control-drive circuit 6 is subjected to pulse-width modulation and applied to the electrodes Y.sub.1, . . . , Y.sub.m+4, . . . at the same time. At the points where the second address electrode X.sub.n with the horizontal scanning voltage applied thereto intersects the first address electrodes Y.sub.m, Y.sub.m+1, Y.sub.m+4 and which include the apertures 41 on the electrode X.sub.n, electron beams are drawn through the apertures 41 while being controlled to irradiate the corresponding phosphor dots.
With reference to FIG. 7 showing the fluorescent screen, the R, G, B phosphor dots 11 are arranged in a black matrix 13 in the delta pattern. When the electron beams are withdrawn straight, the beam spots 14 impinge on the respective dots 11 centrally thereof to produce a sharp image. As will be apparent from FIG. 3, however, during scanning with the nth second address electrode 44, i.e., electrode X.sub.n, the image signal applied to the first address electrodes 42 acts effectively for the electrodes Y.sub.m, Y.sub.m+2, Y.sub.m+4 in controlling the beams but ineffectively for the electrodes Y.sub.m+1, Y.sub.m+3 since no scanning voltage is applied to the second address electrode X.sub.n+1 despite the impression of the image signal voltage on these first address electrodes. Conversely during the next horizontal scanning period, the first address electrodes Y.sub.m+1, Y.sub.m+3 become effective electrodes, and the electrodes Y.sub.m, Y.sub.m+2, Y.sub.m+4 are ineffective.
Because the image signal is applied to the first address electrodes 42 at the same time regardless of the effectiveness, the electron beams drawn through the apertures 41 in the effective electrodes are deflected by being influenced by the image signal voltage on the ineffective electrodes as represented in FIG. 7 by beam spots 14A, 14B failing to fully strike on the phosphor dot and partly impinging on the black matrix, or by a beam spot 14C which is deformed. The deflection of electron beams entails the problem of producing images of lower luminance or reduced sharpness.